Electron discharge device having a barium dispensing anode structure



Oct. 17, 1967 J. E. BEGGS 3,343,092

ELECTRON DlSCHARGE DEVICE HAVING A BARIUM DISPENSING ANODE STRUCTURE Filed Jan. 4, 1965 Mvemor Jam 65 E Beggs,

by %L p? His Affomey- United States Patent O 3,348,092 ELECTRON DISCHARGE DEVICE HAVING A BARIUM DISPENSING AN ODE STRUCTURE James E. Beggs, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Jan. 4, 1965, Ser. No. 423,108 6 Claims. (Cl. 313-346) ABSTRACT OF THE DISCLOSURE An electron discharge device includes a cathode which is non-emissive at the operating temperature of the device. To render the cathode emissive, the anode includes a source of barium so that as the device is heated the anode generates barium vapor which deposits on the cathode to make it emissive, resulting in a device which can be operated at lower temperature and has a longer life than one having a barium material on the cathode itself.

This invention relates to electron discharge devices and particularly to such a device capable of long operation at high current densities.

Oxide coated cathodes are frequently employed in electron discharge devices to provide copious electron emission. In particular, cathodes coated with material including barium oxide and oxides of strontium and calcium produce work functions of about 1.6 electron volts, wherein the cathode base, for example nickle containing certain impurities, reduces the barium oxide leaving free barium as the agent for reducing the work function.

In high temperature, high current density discharge devices it is desired to form the cathode electrode of more refractory metals such as tungsten or molybdenum. These metals react with an oxide coating quite slowly below 800 C. so that an oxide coating can have appreciable life. However, for prolonged life, or operation at temperatures above 800 C., a more stable and quite thin oxide coating is desirable. With such a thin and stable oxide coating, reduction is slow and emission insuflicient.

It is therefore an object of the present invention to provide a high temperature electron discharge device operating at high current density with copious emission of electrons from a highly refractory metal cathode.

In an effort to solve the aforementioned problems, I have discovered that a barium oxide coated or impregnated anode, heated to about 500 C., can dispense barium at a sufficient rate to cause an adjacent cathode thinly coated with strontium-calcium oxide or similar material to emit electrons as copiously as a bariumstrontium oxide cathode. Since strontium-calcium oxide is reduced slowly by the base metal of the cathode as compared with the usual barium-strontium oxide coatings, the cathode coating is stable and can have a long life at normal operating temperatures, or can be operated at a higher temperature with consequent higher emission. A porous layer may be provided at the surface of the anode. By impregnating the barium-oxide coating into the porous material it is protected from direct electron bombardment. The coating may alternatively be positioned on the anode surface back of the grid conductors whereby it is also protected from electron bombardment.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing.

Referring to the single figure of the drawing, illustrating a partial cross-sectional view of an electron discharge device in accordance with the present invention, cathode 1 having a flat face 2 coated with oxide material as hereinafter described, is spaced from a cylindrical anode 4 having an end face 5 which may be formed of porous material. Between the juxtaposed faces of the cathode and the anode is disposed a flat grid 6 separated and insulated from both the anode and the cathode by means of cylindrical spacers 7 and 8 formed of ceramic material forming a part of the outside envelope of the discharge device. The cathode is supported upon a metal cylinder 3 and is heated by means of a heater element located on its underside and connected between heater ead 9 and cylinder 3.

As will be appreciated by those skilled in the art, an electron discharge device cathode can be activated for example by providing an emissive oxide coating therefor. Such coating can be provided by applying on to the surface of the cathode as a slurry, certain alkali earth metal carbonates, a commonly used mixture being approximately equal percentages of strontium and barium carbonate with a small percentage of calcium carbonate. After the device is evacuated the cathode is heated to a temperature at which the carbonates are converted to oxides and at which temperature a portion of the barium oxide is reduced forming free barium. The free barium activates the cathode so that, when heated, electrons will be emitted therefrom. The barium activates the cathode rather than the strontium or calcium since it is relatively the most active and is therefore reduced at a lower temperature, with the strontium and calcium remaining in oxide form.

In high temperature, high performance discharge devices, an oxide coating on the cathode must be quite thin for operation at high current density (one ampere per square centimeter or more) in order to prevent local overheating of the oxide and resultant destructive cathode sparking. On refractory base metals such as molybdenum and tungsten, such thin coatings can last for some time, at temperatures below 800 C. However, to obtain high level emission it is desirable to operate the cathode at temperatures above 800 C. In these cases the coating including barium oxide on tungsten or molybdenum reduces too rapidly. Therefore to obtain a suitably long cathode life particularly at high temperatures, it becomes desirable to find oxide coatings that are more stable and longer lasting.

In accordance with an embodiment of the present invention the cathode, which is appropriately formed to tungsten or molybdenum, has applied thereto strontium and calcium carbonates providing a strontium and calcium oxide coating when heated in the conventional manner as outlined above. However, barium and strontium carbonates are applied to the anode and the anode is heated in the construction of the evacuated device producing barium oxide material. The anode in the discharge device according to the present invention is a refractory metal such as employed for the cathode and is preferably tungsten or molybdenum. However, the anode operates at a lower temperature than the cathode under normal circumstances such that the barium oxide is reduced at a very low rate. The rate of reduction of barium oxide is related to an anode temperature of 600 C. or less rather than a cathode temperature of 800 C. or more. Approximately one one-hundredth as much barium evaporates from the anode as would from a cathode having a customary barium oxide coating. This decreased production of barium greatly increases the life of the device and greatly reduces the formation of electrical leakage across insulators. It also helps decrease the level of emission from the grid. However, sufficient barium is dispensed to the cathode in the present device to provide as much electron emission as if the barium oxide Were located on an ideal cathode.

The oxides used on the cathode, for example as given above, are chosen for their low rate of reduction at cathode operating temperaures, so they will not materially contribute to leakage or grid emission problems, and so that a thin coating layer will havea long life. Basically a cathode oxide coating is desired which will form a stable base for barium dispensed from the anode, resulting in an overall low work function. The work function of the cathode herein described is about 1.6 electron volts; however, the work function of barium on clean metal would be higher, on the order of 2.0 electron volts, at a temperature of approximately 825 C. in each case, thus making the thin oxide base coating on the cathode appear quite desirable.

The coating upon the cathode is stable-during operation of the device as indicated above. It should be very thin (on the order of about A mil) so that its resistance is low and resistance heating and voltage drop are minimized when high level (more than one ampere per square centimeter) emission is being drawn from the cathode. In thecase of the strontium-calcium oxide coating, the oxides are provided, for example each in the proportion of about 50 mole percent. Other thin oxide layers which have a high degree of stability may be employed upon the cathode as well as the oxides of strontium and/or calcium. Alumina and thoria are also appropriate, for example.

The stable cathode coating permits the cathode to be run stably at higher temperatures, resulting in increased current density. The cathode of the present device may be operated at temperatures as high as 9501000 C., still with a reasonable lifetime for the device, with the electron emissive barium being dispensed from the lower temperature anode. At 825 C. saturated emissions of from 5 to amperes per square centimeter are attained from a strontium-calcium oxide coated cathode, while at 950 C. these figures are more than doubled.

The emissive oxide coating dispensed from the anode may be applied thereto as a thin uniform coating on a flat smooth anode surface facing the cathode. For example a thin (five micron or less) coating of bariumstrontium oxide may be so applied. When the anode is operated at low voltage and at a temperature not exceeding 600 C., this coating can last for some time. However, when the anode is operated at a high positive potential, the coating can be dissociated by electronbombardment. To avoid rapid loss of coating in this manner, a porous refractory metal layer may be provided for the face of the anode as at reference numeral 5 in the drawing. Alternatively, a coating of emissive material on the anode may be located immediately behind grid conductors, protecting it from electron bombardment.

The porous layer 5 while preferably juxtaposed opposite the cathode, may also be located elsewhere on the anode or on the anode structure residing at approximately the'same temperature as the anode. The porous layer 5 is made of such thickness that the surface temperature thereof is on the order of 500 C. when the anode is operated at maximum dissipation. At this temperature, barium is dispensed at a rate more than sufficient to keep a strontium-calcium oxide-coated cathode at a high level of activity in a discharge device having a good vacuum environment such as one employing titanium metal parts. In the foregoing example, barium and strontium oxides may be provided each in the proportion of 50 mole percent.

In the illustrated embodiment of the present invention, barium oxide in combination with another oxide is coated upon or impregnated into the discharge device anode, ordinarily in the initial carbonate form. It is appreciated, however, that barium oxide may be similarly employed alone or in combination with other substances.

' It will be apparent to those skilled in the art that many other changes and modifications may be employed without departing from my invention in its broader aspects,

and I therefore intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electron discharge device comprising a cathode having a thin stable oxide coating which is normally non-emissive at the temperature of operation of the said cathode, means for heating said cathode to its operating temperature, and an anode juxtaposed with respect to said cathode which anode is formed of a refractory metal material residing at a temperature somewhat less than said cathode during operation of said device, wherein said anode is provided with barium oxide material reduced very gradually to barium at the operating temperature of said anode for dispensing said barium on to the oxide coating of said cathode for enhancing the electron emission thereof.

2. An electron discharge device comprising a cathode having a thin stable oxide coating which is normally nonemissive at the temperature of operation of the said cathode, means for heating said cathoderto its operating temperature, and an anode spaced from said cathode which anode is formed of a refractory metal selected from the group consisting of tungsten and molybdenum residing at a temperature somewhat less than said cathode during operation of said device, wherein said anode is provided 1 coating which is normally non-emissive at the tempera ture of operation of the saidcathode, means for heating said cathode to an operating temperature above 800 C., and an anode juxtaposed with respect to said cathode which anode is formed of a refractory metal selected from the group consisting of tungsten andmolybdenum residing at a temperature below approximately 600 C. during operation of said device, wherein said anode is provided with barium oxide material reduced very gradually to barium at the operating temperature of said anode for dispensing barium on'to thecoating of said cathode to enhance the electron emission thereof.

4. An electron discharge device comprising a flat cathode formed of a refractory metal and having a normally non-emissive thin stable oxide coating, means for heating said cathodeto an operating temperature above 800 C., and an anode juxtaposed with respect to said cathode, said anode having a porous layer formed of a refractory metal material residing at a temperature somewhat less than said cathode during operation of said device, wherein said layer is impregnated with barium oxide material reduced very gradually to barium at the operating temperature of said anode for dispensing barium on to the oxide coating of said cathode to enhance the electron emission thereof.

5. An electron discharge device comprising a cathode formed of a refractory metal selected from the group consisting of tungsten and molybdenum and provided with a thin stable oxide coating which is normally nonemissive at the temperature of operation of the said cathoxide coating of said cathode to enhance the electron emission thereof.

6. An electron discharge device comprising a cathode formed of a refractory metal selected from the group consisting of tungsten and molybdenum and provided with a normally non-emissive thin stable oxide coating including oxides of strontium and calcium, means for heating said cathode to its operating temperature above 800 C., and an anode juxtaposed with respect to said cathode coating, said anode having a layer opposite said. 10

to barium at said anode for dispensing barium on to the oxide coating of said cathode to enhance the electron emission thereof.

References Cited UNITED STATES PATENTS 3,110,081 11/1963 Hendriks 313346 FOREIGN PATENTS 671,992 5/1952 Great Britain. 1,380,944 10/1964 France.

5 JOHN W. HUCKERT, Primary Examiner.

A. J. JAMES, Assistant Examiner. 

1. AN ELECTRON DISCHARGE DEVICE COMPRISING A CATHODE HAVING A THIN STABLE OXIDE COATING WHICH IS NORMALLY NON-EMISSIVE AT THE TEMPERATURTE OF OPERATION OF THE SAID CATHODE, MEANS FOR HEATING SAID CATHODE TO ITS OPERATING TEMPERATURE, AND AN ANODE JUXTAPOSED WITH RESPECT TO SAID CATHODE WHICH ANODE IS FORMED OF A REFRACTORY METAL MATERIAL RESIDING AT A TEMPERATURE SOMEWHAT LESS THAN SAID CATHODE DURING OPERATION OF SAID DEVICE, WHEREIN SAID ANODE IS PROVIDED WITH A BARIUM OXIDE MATERIAL REDUCED VERY GRADUALLY TO BARIUM AT THE OPERATION TEMPERATURE OF SAID ANODE FOR DISPENSING SAID BARIUM ON TO THE OXIDE COATING OF SAID CATHODE FOR ENHANCING THE ELETRON EMISSION THEREOF. 